(1) Field of the Invention
The invention relates generally to a power control system between source and load as a function of an independent control system.
(2) Description of the Prior Art
When starting multiphase AC induction motors, it is desirable to start the motor with the highest possible torque and with a minimum KVA (kilovolt-ampere) power input. Solid state control circuits have been utilized incorporating SCR's which phase control the AC input power, thus providing high instantaneous power and enabling high starting torques to be achieved. However, in these circuits, it still is necessary to start the motor with the input AC voltage at a high phase angle in order to prevent loss of phase synchronization caused by line noise generating a false signal.
Induction motors, as is well known, run at a full speed which is less than synchronous speed of a truly synchronous motor of the same number of poles at the same power source frequency. This "slip" in speed is related to the torque developed and the load presented as well as to various design factors, although it is seldom more than four or five percent under the corresponding synchronous speed. Nearly all induction motors of more than a few horsepower are three phase units.
Alternate starting techniques and apparatus include full line voltage starting which is applicable to only single and poly phase motors of relatively small horsepower, and a variety of controlled starting devices for reducing the applied voltage at least for a small initial acceleration period.
The controlled starting devices of the prior art include auto-transformer starting, series resistance starting with one or more resistance steps before the full power run condition, and wye-delta switched starting. In the auto-transformer arrangement, the supply voltage, and therefore the KVA input is reduced manually or on a preprogrammed basis until the motor reaches a speed on the order of seventy-five percent of full speed. At that time, the load is effectively cut-over to full voltage. Initial KVA may be only about three times the rated running KVA, but tends to surge to a higher value once the full line voltage is applied. Series resistance starting produces surges at each switching to a lower value of resistance as does the wye-delta configuration when going from delta to wye load connection. That system employs the phase legs of the load first in a wye connection which reduces the initial period KVA to about one third of that resulting from full voltage starting but produces transient demand when the load legs are switched to the delta configuration. Wye-delta and series resistance starting torques are particularly low as compared to auto-transformer starting.
The full voltage starting technique and any of the other aforementioned prior art arrangements including mode switching (stepping) produce mechanical shock in the motor and driven mechanical apparatus, which can have undesirable impact on equipment life and operational effectiveness.
The cost of starting equipment and its maintenance is also a factor in the prior art systems, even the full voltage start systems, because of the required large oversizing of contactors required for all but very small motors. Mechanical switching and programmers are not only expensive to build, but also costly to repair when they malfunction. Motor burn-out because of excessive starting load is also a factor in motor starting design, and cannot be readily dealt with in the aforementioned prior art devices.
Still further, control of electric demand consistent with adequate starting torque, and avoidance of large transient demands from the power system are important from an economic point of view both as an internal wiring matter, but also because of power utility demand-based power rates.
Although solid state motor control devices are known, they do not include automatic motor acceleration "tracking" to provide adequate torque characteristics and to provide a relatively flat KVA demand during starting along with a relatively flat torque curve during acceleration.